Light scanning unit and image forming apparatus employing the same

ABSTRACT

A light scanning unit includes a light source unit emitting a light beam according to an image signal, a light deflector scanning the light beam that is deflectively emitted by the light source unit, a housing having a side portion where the light source unit is provided and a base surface on which the light deflector is provided, and a deformation prevention member connecting opposite sides of the housing across an upper side of the base surface. The light scanning unit is employed by an image forming apparatus.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/307,885, filed on Jun. 18, 2014, which claims the priority benefit ofProvisional U.S. Patent Application No. 61/892,039, filed on Oct. 17,2013, and Korean Patent Application No. 10-2013-0125550, filed on Oct.21, 2013, in the Korean Intellectual Property Office, the disclosures ofwhich are incorporated herein in their entirety by reference.

BACKGROUND

1. Field

One or more embodiments relate to a light scanning unit and an imageforming apparatus employing the same, and more particularly, to a lightscanning unit for reflecting and deflecting luminous flux output from alight source toward a light deflector to form an image on a surface tobe scanned, and an image forming apparatus employing the same andperforming an electrophotographic process.

2. Description of the Related Art

Electrophotographic image forming apparatuses such as laser printers,digital copiers, multifunction printers (MFPs), etc. have a structure inwhich light is scanned onto a photosensitive body by using a lightscanning unit to form an electrostatic latent image, a formedelectrostatic latent image is developed into a developed image by usinga developer such as toner, and the developed image is transferred onto aprinting medium.

The light scanning unit generally uses plastic resin as a material for ahousing for mounting optical parts such as a light source or a lightdeflector. However, the housing formed of a plastic resin material has aproblem in that parts of the housing may be thermally deformed accordingto a change in peripheral and internal temperatures and a position ofthe photosensitive body where an image is formed by a light beam may bedeviated.

The tendency for a change in the image forming position on thephotosensitive body due to the thermal deformation may be stronger inthe light scanning unit for scanning a plurality of light beams such asa color image forming apparatus. In a color image forming apparatus ofthe related art in which a plurality of luminous fluxes are scanned byusing one light deflector, in order to guide the luminous flux afterscanning to each photosensitive body corresponding to a different color,the luminous fluxes are obliquely incident with respect to a plane thatis perpendicular to a rotational shaft of a deflection unit and then theluminous fluxes are split. An oblique incident type light scanning unitis a structure of reducing material costs by making a compact opticalpath layout and reducing the number of parts. In the color image formingapparatus of the related art, to address the thermal deformationproblem, changing tendencies of the image forming positions on thephotosensitive body, which occurs due to heating of light sources and achange in the peripheral temperature, may be congruent to each other byfixing flanges in which light sources are assembled on the housing byusing elastic members and varying pressing-directions of the elasticmembers in which the elastic members are assembled according to theflanges.

However, for a light scanning unit in which a distance between aplurality of light sources is narrow, for example, less than or equal to12 mm, it is difficult to separately accommodate a holder (flange) foreach light source and to make a space for assembling the elastic memberson the holder (flange). Also, as the elastic members are assembled foreach light source, the number of parts and the number of steps areincreased so that the material costs are increased and a process iscomplicated.

SUMMARY

In an aspect of one or more embodiments, there is provided a lightscanning unit which reduces a change in image forming positions of aplurality of light sources, and an image forming apparatus employing thesame.

In an aspect of one or more embodiments, there is provided a lightscanning unit includes a light source to emit a light beam according toan image signal, a light deflector deflectively scanning the light beamthat is emitted by the light source, a housing having a first side wallwhere the light source is provided and a base surface on which the lightdeflector is provided, and a deformation prevention member to connectopposite sides of the housing across an upper side of the base surface.

The deformation prevention member may be formed of a material having athermal expansion coefficient that is lower than that of a material ofthe housing.

The housing may be formed of plastic resin and the deformationprevention member is formed of metal or plastic resin.

The housing may include a second side wall arranged facing the firstside wall with the light deflector interposed between the first andsecond side walls, and the deformation prevention member may connect thefirst and second side walls.

A portion of the deformation prevention member that is connected to thefirst side wall is an area where the light source may be provided or anarea adjacent to the area where the light source may be provided.

The deformation prevention member may have a form of a long bar or aplate.

One or a plurality of the deformation prevention members may be provided

The deformation prevention member may be disposed to be higher than aninstallation height of a light source of the light source.

The deformation prevention member may have a strength reinforcementstructure.

The strength reinforcement structure may include at least one of a bentportion on a cross-sectional surface of the deformation preventionmember, an embossment formed on a surface of the deformation preventionmember, and a rib provided on a surface of the deformation preventionmember.

The housing may further include a cover disposed above the deformationprevention member and the cover may include a heat dissipation holeprovided adjacent to a position where the deformation prevention memberis disposed.

The light source may include a plurality of light sources and theplurality of light sources are arranged at one side wall of the housing.

The light source may further include an integrated light source holderthat fixes the plurality of light sources to the housing.

The light source holder may be integrally formed with the housing.

The light source may further include a circuit board where the pluralityof light sources are mounted.

Light beams emitted by the plurality of light sources may be obliquelyincident on the light deflector with respect to a sub-scanningdirection.

The light source may include first to fourth light sources that emitfirst to fourth light beams, and the light deflector may deflectivelyscan the first and second light beams on one deflection surface and thethird and fourth light beams on a deflection surface that is differentfrom the one deflection surface.

The light scanning unit may further include an incident optical systemthat is arranged between the light source and the light deflector.

The incident optical system may include at least one of a collimatorlens that shapes a light beam emitted by the light source into aparallel luminous flux and a cylindrical lens that focuses the lightbeam, which is emitted by the light source, on a deflection surface ofthe light deflector in a sub-scanning direction.

The light scanning unit may further include an imaging optical systemfor forming on a surface to be scanned an image of a light beam that isdeflectively scanned by the light deflector.

The imaging optical system may include one or more scanning lenses forforming an image of luminous flux at a constant velocity.

In an aspect of one or more embodiments, there is provided anelectrophotographic image forming apparatus which includes an imagereceptor, a light scanner to form an electrostatic latent image byscanning a light beam onto a surface to be scanned of the imagereceptor, and a developer to develop the electrostatic latent imageformed on the image receptor by supplying toner to the electrostaticlatent image, wherein the light scanner includes a light source to emita light beam according to an image signal, a light deflector todeflectively scan the light beam that is emitted by the light source, ahousing having a side portion where the light source is provided and abase surface on which the light deflector is provided and a deformationprevention member to connect opposite sides of the housing across anupper side of the base surface.

In an aspect of one or more embodiments, there is provided a laserscanner which includes a housing having a first sidewall, which isconfigured to receive a light source, and a second sidewall arranged toface the first sidewall; and a deformation prevention member to connectthe first side wall to the second sidewall, wherein the deformationprevention member is formed of a material having a thermal expansioncoefficient that is lower than that of a material of the housing.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects will become apparent and more readilyappreciated from the following description of embodiments, taken inconjunction with the accompanying drawings in which:

FIG. 1 is a perspective view of a light scanning unit according to anembodiment, viewed from the top thereof;

FIG. 2 is a perspective view of the light scanning unit of FIG. 1,viewed from the bottom thereof;

FIG. 3 is a cross-sectional view of the light scanning unit of FIG. 1taken along a line A-A′;

FIG. 4 is a plan view illustrating a light source unit of the lightscanning unit of FIG. 1;

FIG. 5 is a perspective view for explaining the arrangement of a lightdeflector and a light source unit;

FIG. 6 is a perspective view of a deformation prevention member used forthe light scanning unit of FIG. 1;

FIG. 7 illustrates an optical configuration of the light scanning unitof FIG. 1;

FIG. 8 is a schematic view for explaining thermal deformation of a lightscanning unit according to a comparative example;

FIG. 9 is a cross-sectional view for explaining thermal deformation ofthe light scanning unit of FIG. 8;

FIG. 10 is a schematic view for explaining thermal deformation of thelight scanning unit of FIG. 1;

FIG. 11A and FIG. 11B are views for explaining a color registrationerror according to thermal deformation of a light scanning unit;

FIG. 12 is a graph for showing thermal deformation according to atemperature of the light scanning unit of FIG. 8;

FIG. 13 is a graph for showing thermal deformation according to atemperature of the light scanning unit of FIG. 1;

FIG. 14 is a perspective view of a light scanning unit according to anembodiment where a cover is removed, viewed from the top thereof;

FIG. 15 illustrates a deformation prevention member used for the lightscanning unit of FIG. 14;

FIG. 16 is a perspective view of the light scanning unit of FIG. 14,where the cover is closed, viewed from the top thereof; and

FIG. 17 illustrates a schematic structure of an electrophotographic typeimage forming apparatus employing a light scanning unit according to anembodiment.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments, examples of whichare illustrated in the accompanying drawings, wherein like referencenumerals refer to like elements throughout. In this regard, embodimentsmay have different forms and should not be construed as being limited tothe descriptions set forth herein. Accordingly, embodiments are merelydescribed below, by referring to the figures, to explain aspects of thepresent description. As used herein, the term “and/or” includes any andall combinations of one or more of the associated listed items.Expressions such as “at least one of,” when preceding a list ofelements, modify the entire list of elements and do not modify theindividual elements of the list.

The terms used in the present specification are used for explaining aspecific exemplary embodiment, not for limiting the present disclosure.Thus, the expression of singularity in the present specificationincludes the expression of plurality unless clearly specified otherwisein context. Also, the terms such as “comprise” and/or “comprising” maybe construed to denote a certain characteristic, number, step,operation, constituent element, or a combination thereof, but may not beconstrued to exclude the existence of or a possibility of addition ofone or more other characteristics, numbers, steps, operations,constituent elements, or combinations thereof.

Embodiments will now be described more fully with reference to theaccompanying drawings, in which exemplary embodiments are shown.Throughout the drawings, like reference numerals denote like elements.In the following description, when detailed descriptions about relatedwell-known functions or structures are determined to make the gistunclear, the detailed descriptions will be omitted herein.

FIG. 1 is a perspective view of a light scanning unit 100 according toan embodiment, viewed from the top thereof. FIG. 2 is a perspective viewof the light scanning unit 100 of FIG. 1, viewed from the bottomthereof. FIG. 3 is a cross-sectional view of the light scanning unit 100of FIG. 1 taken along a line A-A′.

Referring to FIGS. 1 to 3, the light scanning unit (light scanner) 100according to an embodiment includes a housing 110, a light source unit(light source) 120 and a light deflector 150 mounted in the housing 110,a deformation prevention member 140 provided across the upper portion ofthe housing 110. The housing 110 may be a mold structure formed ofplastic resin. The housing 110 may include a base surface 111 and firstand second side walls 112 and 113 surrounding the base surface 111.

FIG. 4 is a plan view illustrating the light source unit 120 of thelight scanning unit 100 of FIG. 1. FIG. 5 is a perspective view forexplaining the arrangement of the light deflector 150 with respect tothe light source unit 120. Referring to FIG. 4, the light source unit120 includes first to fourth light sources 121 a, 121 b, 121 c, and 121d and a light source holder 125 for fixing the first to fourth lightsources 121 a, 121 b, 121 c, and 121 d on the housing 110. The first tofourth light sources 121 a, 121 b, 121 c, and 121 d may be laser diodes.The first to fourth light sources 121 a, 121 b, 121 c, and 121 drespectively emit first to fourth light beams L1, L2, L3, and L4 thatare modulated according to image signals corresponding to imageinformation about, for example, black (K), magenta (M), yellow (Y), andcyan (C) colors.

In the light scanning unit 100 according to an embodiment, the first tofourth light beams L1, L2, L3, and L4 travel toward the same side of thelight deflector 150, as will be described later with reference to FIG.7. The first to fourth light sources 121 a, 121 b, 121 c, and 121 d maybe provided at the same side wall (hereinafter, referred to as the firstside wall) 112 of the housing 110. Also, since the light scanning unit100 according to an embodiment adopts an oblique optical system as willbe described later, the first to fourth light sources 121 a, 121 b, 121c, and 121 d may be densely arranged in a 2×2 matrix. Accordingly, thelight holder 125 may be integrally formed with respect to the first tofourth light sources 121 a, 121 b, 121 c, and 121 d. In other words,four fixed holes 125 a, 125 b, 125 c, and 125 d are provided in thelight source holder 125 so that the first to fourth light sources 121 a,121 b, 121 c, and 121 d may be inserted in the fixed holes 125 a, 125 b,125 c, and 125 d from the rear surface of the light source holder 125.As illustrated in FIG. 5, viewed from the front surface of the lightsource holder 125, the fixed holes 125 a, 125 b, 125 c, and 125 d, andcorrespondingly, the first to fourth light sources 121 a, 121 b, 121 c,and 121 d, may be arranged symmetrically to the left and right in pairswith respect to a rotational shaft 159 of the light deflector 150. Inother words, the first and second light sources 121 a and 121 b and thethird and fourth light sources 121 c and 121 d are symmetricallyarranged with respect to the rotational shaft 159 of the light deflector150 in a main scanning direction (direction X). The first and secondlight sources 121 a and 121 b may be arranged in parallel in asub-scanning direction (direction Y) and the third and fourth lightsources 121 c and 121 d may also be arranged in parallel in thesub-scanning direction (direction Y). The light source holder 125 andthe first side wall 112 of the housing 110 may be integrally formed of aplastic resin mold. The light source holder 125 may be manufacturedseparate from the housing 110 and attached to the housing 110. The lightsource unit 120 may further include a circuit board 129 where the firstto fourth light sources 121 a, 121 b, 121 c, and 121 d are mounted.

The first to fourth light beams L1, L2, L3, and L4 respectively emittedby the first to fourth light sources 121 a, 121 b, 121 c, and 121 d aredeflectively scanned by a single light deflector that is the lightdeflector 150. The light deflector 150 may include, for example, arotating multi-facet mirror 151 having a plurality of reflectionsurfaces, that is, deflection surfaces, rotating around a rotationshaft, and a drive motor 155 for rotating the rotating multi-facetmirror 151. The light deflector 150 may be mounted on a board 158 andthe board 158 may be installed at an approximately center portion of thebase surface 111. Another example of the light deflector 150 may be amicroelectromechanical system (MEMS).

FIG. 6 is a perspective view of the deformation prevention member 140used for the light scanning unit 100 of FIG. 1. Referring to FIGS. 1 and6, the light scanning unit 110 of an embodiment further includes thedeformation prevention member 140 that connects the first and secondside walls 112 and 113 of the housing 110 across above the base surface111 of the housing 110. A portion of the deformation prevention member140 connected to the first side wall 112 may be an area where the lightsource unit 120 is provided or an area adjacent to the light source unit120 (area R of FIG. 3). The second side wall 113 may be a side wallcorresponding to the first side wall 112 with the light deflector 150interposed therebetween. In this case, the deformation prevention member140 may cross the upper side of the light deflector 150 or an areaadjacent to the upper side of the light deflector 150.

In the light scanning unit 100, the first to fourth light sources 121 a,121 b, 121 c, and 121 d of the light source unit 120 or the drive motorof the light deflector 150 generate heat during operation. The lightscanning unit 100 may be sealed to prevent a malfunction due to dustscattering in the image forming apparatus. Accordingly, the housing 110may be thermally deformed by the heat generated by the light source unit120 and the light deflector 150 mounted in the housing 110 or by thesurrounding environment. As described later, the thermal deformation ofthe housing 110 may be restricted by the deformation prevention member140.

The deformation prevention member 140 may be formed of a material havinga thermal expansion coefficient less than that of the housing 110. Forexample, the deformation prevention member 140 may be formed of metal orplastic resin. The deformation prevention member 140 has an elongatedbar structure 141 having opposite ends that are fixed to the first andsecond side walls 112 and 113 of the housing 110 at four points by usingcoupling units (couplers) such as screws. The deformation preventionmember 140 may be disposed to be higher than a height at which the firstto fourth light sources 121 a, 121 b, 121 c, and 121 d of the lightsource unit 120 are provided, with respect to the base surface 111 ofthe housing 110. The deformation prevention member 140 may have astrength reinforcement structure. The strength reinforcement structureis used to reinforce the strength of the deformation prevention member140 against the thermal deformation of the housing 110 and may adopt awell-known structure. For example, the strength reinforcement structuremay be a structure having a bent portion 142 formed on a cross-sectionalsurface of the elongated bar structure 141, as illustrated in FIG. 6. Inanother embodiment, the strength reinforced structure may be a ribprovided on a surface of the elongated bar structure 141 of thedeformation prevention member 140, an embossment formed on a surface ofthe elongated bar structure 141 of the deformation prevention member140, or a combination thereof.

Although the light scanning unit 100 according to an embodiment isdescribed such that the deformation prevention member 140 is arranged inan area where the light source unit 120 is provided or an area adjacentthereto (area R), to be deviated to one side, the deformation preventionmember 140 may be arranged directly above the light deflector 150. Also,although the light scanning unit 100 according to an embodiment isdescribed to include only one deformation prevention member 140, aplurality of deformation prevention members 140 may be provided in thearea where the light source unit 120 is provided or the area adjacentthereto (area R). It is not excluded that the deformation preventionmember 140 may be additionally provided outside the area where the lightsource unit 120 is provided or the area adjacent thereto (area R).

FIG. 7 illustrates an optical configuration of the light scanning unitof FIG. 1. An optical system of the light scanning unit 100 according toan embodiment is described below with reference to FIG. 7.

An incident optical system 130 may be provided on an optical pathbetween the first to fourth light sources 121 a, 121 b, 121 c, and 121 dand the light deflector 150. The incident optical system 130 may includea plurality of collimator lenses 131 and a plurality of cylindricallenses 135 provided on an optical path of each of the first to fourthlight beams L1, L2, L3, and L4. The collimator lenses 131 are focusinglenses that make the first to fourth light beams L1, L2, L3, and L4respectively emitted by the first to fourth light sources 121 a, 121 b,121 c, and 121 d parallel light beams or convergent light beams. Asillustrated in FIG. 5, the collimator lenses 131 are arranged in frontof the fixed holes 125 a, 125 b, 125 c, and 125 d of the light sourceholder 125. A lens holder of the collimator lenses 131 may be integrallyformed with the light source holder 125. The cylindrical lenses 135 areanamorphic lenses that focus the first to fourth light beams L1, L2, L3,and L4 in a direction corresponding to the sub-scanning direction(direction Y) to form images of the first to fourth light beams L1, L2,L3, and L4 almost linearly on a deflection surface of the lightdeflector 150.

As illustrated in FIGS. 4 and 5, since the first and second lightsources 121 a and 12 b are arranged close to each other in thesub-scanning direction (direction Y) and the third and fourth lightsources 121 c and 12 d are arranged close to each other in thesub-scanning direction (direction Y), one cylindrical lens 135 may becommonly used with respect to the first and second light beams L1 and L2and the other cylindrical lens 135 may be commonly used with respect tothe third and fourth light beams L3 and L4. The cylindrical lenses 135may be separately provided for each of the first to fourth light beamsL1, L2, L3, and L4. In some cases, the collimator lenses 131 and thecylindrical lenses 135 may be functionally replaced with one opticalpart for each optical path. An aperture stop (not shown) may be furtherprovided on an optical path of each of the first to fourth light beamsL1, L2, L3, and L4. The aperture stop shapes a section of a luminousflux, that is, the diameter and the shape, of each of the first tofourth light beams L1, L2, L3, and L4.

The incident optical system 130 may be arranged such that the first tofourth light beams L1, L2, L3, and L4 emitted by the first to fourthlight sources 121 a, 121 b, 121 c, and 121 d are obliquely incident withrespect to deflection surfaces of the light deflector 150. For example,the first light source 121 a is disposed above the second light source121 b in the sub-scanning direction (direction Y) so that the firstlight beam L1 may be obliquely incident on one deflection surface of thelight deflector 150 at an incident angle θ with respect to the normaldirection of the deflection surface, and the second light source 121 bis disposed under the first light source 121 a in the sub-scanningdirection (direction Y) so that the second light beam L2 may beobliquely incident on the same deflection surface of the light deflector150 at the incident angle θ with respect to the normal direction of thedeflection surface. Similarly, the third light source 121 c is disposedabove the fourth light source 121 d in the sub-scanning direction(direction Y) so that the third light beam L3 may be obliquely incidenton other deflection surface of the light deflector 150 at an incidentangle θ with respect to the normal direction of the other deflectionsurface, and the fourth light source 121 d is disposed under the thirdlight source 121 c in the sub-scanning direction (direction Y) so thatthe fourth light beam L4 may be obliquely incident on the otherdeflection surface of the light deflector 150 at the incident angle θwith respect to the normal direction of the other deflection surface.The incident angle θ of the first to fourth light beams L1, L2, L3, andL4 may be set within a range of, for example, about 2° to 4°. As theincident optical system is designed as an oblique optical system, thecylindrical lenses 135 that are described above or first scanning lenses161 ab and 161 cd that will be described later are commonly used so thatthe number of optical parts may be reduced and thus costs for materialsmay be reduced and the light scanning unit 100 may be made compact.

An imaging optical system 160 may be provided on an optical path betweenthe light deflector 150 and first to fourth photosensitive drums 310.The imaging optical system 160 forms images of the first to fourth lightbeams L1, L2, L3, and L4 deflectively scanned by the light deflector 150on each of outer circumferential surfaces, that is, surfaces to bescanned, of the first to fourth photosensitive drums 310.

The imaging optical system 160 may include lenses having an fθcharacteristic of correcting the first to fourth light beams L1, L2, L3,and L4 and scanned at a constant velocity onto the first to fourthphotosensitive drums 310. In an example, the imaging optical system 160may include the first scanning lenses 161 ab and 161 cd and secondscanning lenses 165 a, 165 b, 165 c, and 165 d, which are provided on anoptical path of each of the first to fourth light beams L1, L2, L3, andL4. The first scanning lenses 161 ab and 161 cd may be designed to haverefractive powers of almost zero (0) in the sub-scanning direction,whereas the second scanning lenses 165 a, 165 b, 165 c, and 165 d may bedesigned to have desired refractive powers in the sub-scanningdirection. The second scanning lenses 165 a, 165 b, 165 c, and 165 dclosest to the surface to be scanned may be eccentrically arranged suchthat a light beam may deflectively pass in the sub-scanning directionwith respect to the apex of the lens.

The first scanning lens 161 ab may be commonly used for the first andsecond light beams L1 and L2 that are deflectively scanned andparallelly separated from each other in the sub-scanning direction. Theother first scanning lens 161 cd may be commonly used for the third andfourth light beams L3 and L4 that are deflectively scanned andparallelly separated from each other in the sub-scanning direction. Assuch, as the first scanning lenses 161 an and 161 cd are commonly used,the number of optical parts may be reduced and thus the light scanningunit 100 may be made compact. A first scanning lens may be independentlyprovided for each of the first to fourth light beams L1, L2, L3, and L4.Also, although the imaging optical system 100 includes two scanninglenses for each optical path in an embodiment, one scanning lens orthree or more scanning lenses may be provided for each optical path.

An optical path change member 170 may be provided on the optical pathsof the first to fourth light beams L1, L2, L3, and L4. For example, theoptical path change member 170 may include first reflection mirrors 171a, 171 b, 171 c, and 171 d arranged between the first scanning lenses161 ab and 161 cd and the second scanning lenses 165 a, 165 b, 165 c,and 165 d and second reflection mirrors 175 a, 175 b, 175 c, and 175 darranged after the second scanning lenses 165 a, 165 b, 165 c, and 165d. The optical path change member 170 may fold the optical paths of thefirst to fourth light beams L1, L2, L3, and L4 so that the lightscanning unit 100 is compacted.

A sync detection optical system (not shown) for detecting sync signalsof the first to fourth light beams L1, L2, L3, and L4 that aredeflectively scanned by the light deflector 150 may be provided. Next,the function of the deformation prevention member 140 in the lightscanning unit 100 according to an embodiment will be described below incomparison with a light scanning unit 100′ according to a comparativeexample.

FIG. 8 is a schematic view for explaining thermal deformation of thelight scanning unit 100′ according to a comparative example. FIG. 9 is across-sectional view for explaining thermal deformation of the lightscanning unit 100′ of FIG. 8. FIG. 10 is a schematic view for explainingthermal deformation of the light scanning unit 100 of FIG. 1.

Referring to FIGS. 8 and 9, a structure of the light scanning unit 100′according to the comparative example is substantially the same as thatof the light scanning unit 100 of an embodiment, except that thedeformation prevention member 140 is not provided.

Referring to FIGS. 8 and 9, the heat generated during the operation ofthe light scanning unit 100′ according to the comparative example mainlyconcentrates on the light source unit 120 and other heat is generated bythe light deflector 150. Due to the heat, bending 140′ is generated suchthat a center portion of the base surface 111 of the housing 110 is bentupwardly compared to a reference base surface 118, that is, a basesurface before thermal deformation, and the first and second side walls112 and 113 are bent outwardly with respect to reference side walls 119,that is, side walls before thermal deformation. As a result of the abovethermal deformation, since the first to fourth light sources 121 a, 121b, 121 c, and 121 d are densely arranged in a matrix of 2×2, the firstto fourth light sources 121 a, 121 b, 121 c, and 121 d are oblique inthe same direction. Since optical paths of the first and second lightbeams L1 and L2 scanned by the first and second light sources 121 a and121 b disposed at the left side with respect to the rotational shaft 159of the light deflector 150 and optical paths of the third and fourthlight beams L3 and L4 scanned by the third and fourth light sources 121c and 121 d disposed at the right side with respect to the rotationalshaft 159 of the light deflector 150 are different to each other, atendency of a change of the imaging positions of the first and secondlight beams L1 and L2 due to the thermal deformation and that of thethird and fourth light beams L3 and L4 due to the thermal deformationappear to be opposite, as will be described later with reference to FIG.11A and FIG. 11B.

Referring to FIG. 10, the light scanning unit 100 according to anembodiment employing the deformation prevention member 140 disposedacross the upper portion of the housing 110 may reduce thermaldeformation of the housing 110. In other words, as illustrated in FIGS.8 and 9, when the deformation prevention member 140 does not exist, thebending 140′ of the housing 110 due to thermal deformation is generatedsuch that the center portion of the base surface 111 is bent upwardlywith respect to the reference surface, that is, a surface before thermaldeformation, and the first and second side walls 112 and 113 are bentoutwardly. Accordingly, as illustrated in FIGS. 8 and 9, as thedeformation prevention member 140 is provided across the upper portionof the housing 110, connecting the area of the first side wall 112 wherethe light source unit 120 is provided and having the most thermal stressor the area adjacent to the light source unit 120 (area R) and thesecond side wall 113 opposite thereto, the bending 140′ of the housing110 may be reduced.

FIG. 11A and FIG. 11B are views for explaining a color registrationerror according to thermal deformation of a light scanning unit. FIG. 12is a graph for showing thermal deformation according to a temperature ofthe light scanning unit 100′ of FIG. 8. FIG. 13 is a graph for showingthermal deformation according to a temperature of the light scanningunit 100 of FIG. 1.

Referring to FIG. 11A, when there is no color registration, four lightbeams corresponding to the black (K), magenta (M), yellow (Y), and cyan(C) colors are scanned at accurate positions. However, as described withreference to FIGS. 8 and 9, the bending 140′ due to thermal deformationcauses disarrangement of the optical parts including the light sourceunit 120 and thus, as illustrated in FIG. 11B, the four light beamscorresponding to the black (K), magenta (M), yellow (Y), and cyan (C)colors is scanned incorrectly onto the surface to be scanned.Accordingly, the quality of a color image formed on a print medium P asabove is deteriorated.

A relatively large amount of deterioration of the image quality due to acolor registration error is generated as the temperature graduallyincreases in the light scanning unit 100′ according to a comparativeexample, as illustrated in FIG. 12. In contrast, in the light scanningunit 100 according to an embodiment, relatively less deterioration ofthe image quality due to a color registration error is generated, eventhough the temperature gradually increases. For example, in FIGS. 12 and13, a thick solid line Y_Var denotes a sum of errors with respect to theblack (K), magenta (M), yellow (Y), and cyan (C) colors. When thetemperature increases by 6° from the room temperature of 25° in a statein which no color registration exists, the amount of deformation in thelight scanning unit 100′ according to a comparative example is about 230μm, whereas the amount of deformation in the light scanning unit 100according to an embodiment is about 110 μm. Accordingly, it can be seenthat the deformation prevention member 140 may reduce the colorregistration error due to thermal deformation to be less than or equalto 50%.

FIG. 14 is a perspective view of a light scanning unit 200 according toan embodiment, where a cover is removed, viewed from the top thereof.FIG. 15 illustrates a deformation prevention member 240 used for thelight scanning unit 200 of FIG. 14. Since the constituent elements ofthe light scanning unit 200 according to an embodiment are substantiallythe same as those of the above-described light scanning unit 100 exceptfor the deformation prevention member 240, the following descriptionwill focus on differences between the light scanning unit 200 accordingto an embodiment and the light scanning unit 100 according to anembodiment.

Referring to FIGS. 14 and 15, the deformation prevention member 240 mayhave a form of a plate 241 having a large width. As in theabove-described embodiment, the deformation prevention member 240 mayconnect the first side wall 112 on which the light source unit 120 isprovided and the second side wall 113 corresponding to the first sidewall 112 with the light deflector 150 interposed therebetween. In otherwords, opposite ends of the deformation prevention member 240 may befixed to the first and second side walls 112 and 113 at four points byusing a coupling unit (not shown) such as a screw. Furthermore, sincethe deformation prevention member 240 according to an embodiment has theplate 241 having a large width, one side of the deformation preventionmember 240 may cover all the area of the first side wall 112 where thelight source unit 120 is provided or the area adjacent to the lightsource unit 120 (area R). Also, the deformation prevention member 240may be arranged across the upper side of the light deflector 150. Thedeformation prevention member 240 may be provided to cover not only theupper side of the light deflector 150 but also an area where the secondand third light beams L2 and L3 of FIG. 3 that exit to the outside. Inthis case, windows 242 b and 242 c may be provided in the deformationprevention member 240 so that the second and third light beams L2 and L3may exit to the outside.

As in the above-described embodiment, the deformation prevention member240 may have a strength reinforcement structure 243. For example, thestrength reinforcement structure may be a structure having a bentportion 243 formed on a cross-sectional surface of the plate 241 asillustrated in FIG. 15, a rib provided on a surface of the plate 241, anembossment formed on a surface of the plate 241, or a combinationthereof.

FIG. 16 is a perspective view of the light scanning unit 200 of FIG. 14,where the cover is closed, viewed from the top thereof. Referring toFIG. 16, the upper portion of the housing 110 is closed by the cover220. A heat radiation hole 221 may be provided at a position adjacent tothe deformation prevention member 240, with the windows for exiting thefirst to fourth light beams L1, L2, L3, and L4 to the outside. Thenumber or shape of the heat radiation hole 221 is not limited to theabove example.

Although in the above-described embodiments one light source holder 126and one circuit board 129 are provided with respect to the light sourceunit 120 having the first to fourth light sources 121 a, 121 b, 121 c,and 121 d, the light source holder 126 and the circuit board 129 may beprovided individually or in pairs for each of the first to fourth lightsources 121 a, 121 b, 121 c, and 121 d.

Although in the above-described embodiments the first and second lightsources 121 a and 121 b and the third and fourth light sources 121 c and121 d are arranged together at one side of the light deflector 150, thefirst and second light sources 121 a and 121 b and the third and fourthlight sources 121 c and 121 d may be arranged symmetrically with respectto the light deflector 150. In other words, the first and second lightsources 121 a and 121 b may be arranged at the first side wall 112 ofthe housing 110 and the third and fourth light sources 121 c and 121 dmay be arranged at the second side wall 113 of the housing 110.

FIG. 17 illustrates a schematic structure of an electrophotographic typeimage forming apparatus employing a light scanning unit according to anembodiment. The image forming apparatus of FIG. 17 is a dryelectrophotographic image forming apparatus that prints a color image byusing a dry developer (hereinafter, referred to as toner).

The image forming apparatus according to an embodiment includes thelight scanning unit 100 or 200, a developing unit (developer) 300, anintermediate transfer belt 400, first and second transfer rollers 410and 420, and a fusing unit (fuser) 500, which are accommodated in acabinet 600.

The light scanning unit 100 or 200 for scanning a plurality of lightbeams may be one of the light scanning units according to theabove-described embodiments of FIGS. 1 to 16. For example, the lightscanning unit 100 or 200 may scan four light beams corresponding to theblack (K), magenta (M), yellow (Y), and cyan (C) colors.

The developing unit 300 may be provided for each color corresponding tothe light beams. For example, one developing unit 300 may be providedfor each of the black (K), magenta (M), yellow (Y), and cyan (C) colors.The developing unit 300 includes one of the first to fourthphotosensitive drums 310, which is an image receptor where anelectrostatic latent image is formed, and a developing roller 320 fordeveloping the electrostatic latent image, for each color.

The photosensitive drum 310 is an example of the image receptor, inwhich a photosensitive layer is formed on an outer circumferentialsurface of a cylindrical metal pipe. The outer circumferential surfaceof the photosensitive drum 310 is a surface to be scanned. Thephotosensitive drum 310 is exposed to the outside of the developing unit300 and separated from each other in the sub-scanning direction. Aphotosensitive belt type image receptor may be employed instead of thephotosensitive drum 310.

A charge roller 330 is provided at an upper stream of a region that isexposed to light by the light scanning unit 100 or 200 on the outercircumferential surface of the photosensitive drum 310. The chargeroller 330 is an example of a charger that charges a surface of thephotosensitive drum 310 to a uniform electric potential while rotatingin contact with the outer circumferential surface of the photosensitivedrum 310. A charging bias is applied to the charge roller 330. A coronacharger (not shown) may be used instead of the charge roller 330. Thedeveloping roller 320 having toner adhering to an outer circumferentialsurface thereof supplies the toner to the photosensitive drum 310. Adeveloping bias to supply the toner to the photosensitive drum 310 isapplied to the developing roller 320. Although it is not illustrated inFIG. 17, a supply roller (not shown) adhering the toner accommodated inthe developing unit 300 to the developing roller 320, a restriction unit(not shown) restricting an amount of the toner adhering to thedeveloping roller 320, and an agitator (not shown) transferring thetoner accommodated therein toward the supply roller and/or developingroller 320 may be further provided in each developing unit 300.

The intermediate transfer belt 400 faces the outer circumferentialsurface of the photosensitive drum 310 that is exposed to the outside ofthe developing unit 300. The intermediate transfer belt 400 is anexample of an intermediate transfer body that transfers the toner imageof the photosensitive drum 310 to a print medium P. An intermediatetransfer drum (not shown) may be used as the intermediate transfer bodyinstead of the intermediate transfer belt 400. The intermediate transferbelt 400 circulates in contact with the photosensitive drum 310. Thefour first transfer rollers 410 are each provided at a position facingeach photosensitive drum 310 with the intermediate transfer belt 400interposed therebetween. A first transfer bias is applied to each of thefirst transfer rollers 410 so that the toner image of the photosensitivedrum 310 may be transferred to the intermediate transfer belt 400.

The second transfer roller 420 is arranged to face the intermediatetransfer belt 400 so that the print medium P may pass between the secondtransfer roller 420 and the intermediate transfer belt 400. A secondtransfer bias is applied to the second transfer roller 420 so that thetoner image of the intermediate transfer belt 400 may be transferred tothe print medium P.

A process of forming a color image according to the above-describedstructure will be described below.

The photosensitive drum 310 of the developing unit 300 is charged to auniform electric potential by the charge bias applied to the chargeroller 330. The light scanning unit 100 or 200 exposes the outercircumferential surface of the photosensitive drum 310 in a lengthwisedirection, that is, the main scanning direction. The outercircumferential surface of the photosensitive drum 310 is moved in thesub-scanning direction according to the rotation of the photosensitivedrum 310. Accordingly, a two-dimensional electrostatic latent imagecorresponding to the image information about each of the black (K),magenta (M), yellow (Y), and cyan (C) colors is formed on the outercircumferential surface of each of the four photosensitive drums 310.The sub-scanning direction is perpendicular to the main scanningdirection. The four developing units 300 each supply toner of the black(K), magenta (M), yellow (Y), and cyan (C) colors to the photosensitivedrums 310 to form toner images of the black (K), magenta (M), yellow(Y), and cyan (C) colors.

The toner images of the black (K), magenta (M), yellow (Y), and cyan (C)colors formed on the photosensitive drums 310 are overlapped with oneanother on the intermediate transfer belt 400 by the first transfer biasapplied to the first transfer roller 410, thereby forming a color tonerimage.

A medium for finally accommodating toner, for example, the print mediumP, is transferred by a pickup roller 610 and a transfer roller 620 andinserted between the intermediate transfer belt 400 and the secondtransfer roller 420. The color toner image transferred to theintermediate transfer belt 400 is transferred to the print medium P bythe second transfer bias applied to the second transfer roller 420. Thecolor toner image transferred to the print medium P is retained on asurface of the print medium P by an electrostatic force. The printmedium P to which the color toner image is transferred is sent to thefusing unit 500. The color toner image transferred to the print medium Preceives heat and pressure at a fusing nip (not shown) and is fixedlyfused on the print medium P. The print medium P having been completelyfused is ejected to the outside of the image forming apparatus by aneject roller 630.

The image forming apparatus according to an embodiment is described toform a color image, but the present disclosure is not limited thereto.For example, when a monochromatic image in black and white is to beformed, the light scanning unit 100 or 200 scans a single light beam andthe developing unit 320 may be provided only for the single light beam.Furthermore, in the image forming apparatus according to an embodiment,other constituent elements, except for the light scanning unit 100 or200, that is, the developing unit 300, the intermediate transfer belt400, the first and second transfer rollers 410 and 420, and the fusingunit 500, are described as an example of a printing unit fortransferring a toner image to a print medium by an electrophotographicmethod. Any well-known printing unit may be employed for the imageforming apparatus according to an embodiment. The image formingapparatus may be a laser beam printer, a digital copier, a multifunctionprinter (MFP), etc.

It should be understood that exemplary embodiments described hereinshould be considered in a descriptive sense only and not for purposes oflimitation. Descriptions of features or aspects within each embodimentshould typically be considered as available for other similar featuresor aspects in other embodiments.

While one or more embodiments have been described with reference to thefigures, it will be understood by those of ordinary skill in the artthat various changes in form and details may be made therein withoutdeparting from the spirit and scope of the present disclosure as definedby the following claims.

What is claimed is:
 1. An electrophotographic image forming apparatus comprising: an image receptor; a light scanner to form an electrostatic latent image by scanning a light beam onto a surface to be scanned of the image receptor; and a developer to develop the electrostatic latent image formed on the image receptor by supplying toner to the electrostatic latent image, wherein the light scanner comprises: a light source to emit a light beam according to an image signal; a light deflector to deflectively scan the light beam that is emitted by the light source; a housing having a side portion where the light source is provided and a base surface on which the light deflector is provided; and a deformation prevention member to connect opposite sides of the housing across an upper side of the base surface, and wherein the deformation prevention member is disposed to be higher than an installation height of the light source.
 2. The electrophotographic image forming apparatus of claim 1, wherein the deformation prevention member is formed of a material having a thermal expansion coefficient that is lower than that of a material of the housing.
 3. The electrophotographic image forming apparatus of claim 2, wherein the housing is formed of plastic resin and the deformation prevention member is formed of metal or plastic resin.
 4. The electrophotographic image forming apparatus of claim 1, wherein the housing has a second side wall arranged facing the first side wall with the light deflector interposed between the first and second side walls, and the deformation prevention member connects the first and second side walls.
 5. The electrophotographic image forming apparatus of claim 4, wherein a portion of the deformation prevention member that is connected to the first side wall is an area where the light source is provided or an area adjacent to the area where the light source is provided.
 6. The electrophotographic image forming apparatus of claim 1, wherein the deformation prevention member has a form of a long bar or a plate.
 7. The electrophotographic image forming apparatus of claim 1, wherein one or a plurality of the deformation prevention members are provided.
 8. The electrophotographic image forming apparatus of claim 1, wherein the deformation prevention member has a strength reinforcement structure.
 9. The electrophotographic image forming apparatus of claim 1, wherein the strength reinforcement structure comprises at least one of a bent portion on a cross-sectional surface of the deformation prevention member, an embossment formed on a surface of the deformation prevention member, and a rib provided on a surface of the deformation prevention member.
 10. The electrophotographic image forming apparatus of claim 1, wherein the housing further comprises a cover disposed above the deformation prevention member and the cover comprises a heat dissipation hole provided adjacent to a position where the deformation prevention member is disposed.
 11. The electrophotographic image forming apparatus of claim 1, wherein the light source comprises a plurality of light sources and the plurality of light sources are arranged at one side wall of the housing.
 12. The electrophotographic image forming apparatus of claim 11, wherein the light source further comprises an integrated light source holder that fixes the plurality of light sources to the housing.
 13. The electrophotographic image forming apparatus of claim 12, wherein the light source holder is integrally formed with the housing.
 14. The electrophotographic image forming apparatus of claim 12, wherein the light source further comprises a circuit board where the plurality of light sources are mounted.
 15. The electrophotographic image forming apparatus of claim 11, wherein light beams emitted by the plurality of light sources are obliquely incident on the light deflector with respect to a sub-scanning direction.
 16. The electrophotographic image forming apparatus of claim 1, wherein the light source comprises first to fourth light sources that emit first to fourth light beams, and the light deflector deflectively scans the first and second light beams on one deflection surface and the third and fourth light beams on a deflection surface that is different from the one deflection surface.
 17. The electrophotographic image forming apparatus of claim 1, further comprising an incident optical system that is arranged between the light source and the light deflector.
 18. The electrophotographic image forming apparatus of claim 17, wherein the incident optical system comprises at least one of a collimator lens that shapes a light beam emitted by the light source unit into a parallel luminous flux and a cylindrical lens that focuses the light beam, which is emitted by the light source, on a deflection surface of the light deflector in a sub-scanning direction.
 19. The electrophotographic image forming apparatus of claim 1, further comprising an imaging optical system for forming on a surface to be scanned an image of a light beam that is deflectively scanned by the light deflector.
 20. The electrophotographic image forming apparatus of claim 19, wherein the imaging optical system comprises one or more scanning lenses for forming an image of luminous flux at a constant velocity. 